Walk-regular graph

Short description: Mathematical Graph

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In graph theory, a walk-regular graph is a simple graph where the number of closed walks of any length ℓ from a vertex to itself does only depend on ℓ but not depend on the choice of vertex. Walk-regular graphs can be thought of as a spectral graph theory analogue of vertex-transitive graphs. While a walk-regular graph is not necessarily very symmetric, all its vertices still behave identically with respect to the graph's spectral properties.

## Contents

* 1 Equivalent definitions
* 2 Examples
* 3 Properties
* 4 k-walk-regular graphs
* 5 References

## Equivalent definitions

Suppose that G is a simple graph. Let A denote the adjacency matrix of G, V(G) denote the set of vertices of G, and ΦG−v(x) denote the characteristic polynomial of the vertex-deleted subgraph G−v for all v∈V(G).Then the following are equivalent:

* G is walk-regular.
* Ak is a constant-diagonal matrix for all k≥0.
* ΦG−u(x)=ΦG−v(x) for all u,v∈V(G).

## Examples

* The vertex-transitive graphs are walk-regular.
* The semi-symmetric graphs are walk-regular.[citation needed]
* The distance-regular graphs are walk-regular. More generally, any simple graph in a homogeneous coherent algebra is walk-regular.
* A connected regular graph is walk-regular if: ** It has at most four distinct eigenvalues.
* It is triangle-free and has at most five distinct eigenvalues.
* It is bipartite and has at most six distinct eigenvalues.

## Properties

* A walk-regular graph is necessarily a regular graph, since the number of closed walks of length two starting at any vertex is twice the vertex's degree.
* Complements of walk-regular graphs are walk-regular. [1]
* Cartesian products of walk-regular graphs are walk-regular.[1]
* Categorical products of walk-regular graphs are walk-regular.[1]
* Strong products of walk-regular graphs are walk-regular. [1]
* In general, the line graph of a walk-regular graph is not walk-regular.

## k-walk-regular graphs

A graph is k-walk-regular if for any two vertices v and w of distance at most k, the number of walks of length ℓ from v to w depends only on k and ℓ.[2]

The class of 0-walk-regular graphs is exactly the class of walk-regular graphs

In analogy to walk-regular graphs generalizing vertex-transitive graphs, 1-walk-regular graphs can be thought of as generalizing symmetric graphs, that is, graphs that are both vertex- and edge-transitive. For example, the Hoffman graph is 1-walk-regular but not symmetric.

If k is at least the diameter of the graph, then the k-walk-regular graphs coincide with the distance-regular graphs. In fact, if k≥2 and the graph has an eigenvalue of multiplicity at most k (except for eigenvalues d and −d, where d is the degree of the graph), then the graph is already distance-regular.[3]

## References

1. ↑ 1.0 1.1 1.2 1.3 Godsil, C.D.; McKay, B.D. (April 1980). "Feasibility conditions for the existence of walk-regular graphs". Linear Algebra and Its Applications 30: 51–61. doi:10.1016/0024-3795(80)90180-9.
2. ↑ Dalfó, C.; Fiol, M.A.; Garriga, E. (August 2009). "On t-Cliques in k-Walk-Regular Graphs". Electronic Notes in Discrete Mathematics 34: 579–584. doi:10.1016/j.endm.2009.07.096.
3. ↑ Cámara, Marc; van Dam, Edwin R.; Koolen, Jack H.; Park, Jongyook (November 2013). "Geometric aspects of 2-walk-regular graphs". Linear Algebra and Its Applications 439 (9): 2692–2710. doi:10.1016/j.laa.2013.07.028.

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